[meteorite-list] Deep Impact Comet May Have Formed in Giant Planets Region

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Sep 20 11:47:44 2005
Message-ID: <200509201546.j8KFkHG04461_at_zagami.jpl.nasa.gov>

http://www.nasa.gov/centers/goddard/news/topstory/2005/deepimpact-090905.html

Bill Steigerwald
NASA Goddard Space Flight Center

Deep Impact Comet May Have Formed in Giant Planets Region
September 19, 2005

Comet Tempel-1 may have been born in the region of the solar system
occupied by Uranus and Neptune today, according to one possibility from
an analysis of the comet's debris blasted into space by NASA's Deep
Impact mission. If correct, the observation supports a wild scenario for
the solar system's youth, where the planets Uranus and Neptune may have
traded places and scattered comets to deep space.

"Our observation is a definitive investigation revealing the composition
of comet Tempel-1," said Dr. Michael Mumma of NASA's Goddard Space
Flight Center, Greenbelt, Md. Mumma and his team used the powerful Keck
telescope on top of Mauna Kea, Hawaii, to analyze in great detail light
emitted by Tempel-1 gas ejected by the impact. Because each type of atom
and molecule emits light at unique colors (frequencies), the team was
able to determine the comet's chemical composition by separating its
light into its component colors with an instrument called a
spectrometer. Mumma is lead author of a paper on this research that
appeared in Science Express on Sept.15, 2005.

Comets are chunks of ice and dust that zoom around the solar system in
elongated orbits. This "dirty snowball" is the nucleus of the comet.
Comet nuclei are thought to be cosmic leftovers, condensed remains of
the gas and dust cloud that formed the solar system. As a comet gets
close to the sun, solar heat liberates gas and dust from the nucleus,
forming the coma, which is an extensive, bright cloud around the
nucleus, and one or more tails.

Repeated solar heating can remove materials that have low freezing
temperatures from the surface, giving the comet a crust that's different
chemically from its interior. This makes it hard to discover a comet's
true composition by simply looking at gas that's evaporating from the
surface. NASA's Deep Impact mission crashed into comet Tempel-1 July 4,
2005, allowing scientists to test whether material ejected from its
protected interior was closer to pristine.

By observing Tempel-1 before, during, and after impact, the team was
able to distinguish surface gas from the impact debris, and they
discovered that the interior does indeed have a different chemistry.
"The amount of ethane (C2H6) in the cloud around the comet was
significantly higher after impact than before," said Mumma.

There are two possible explanations for this. In the first, the surface
crust is different from the interior due to solar heating. The interior,
however, is all the same. In the second, the interior is a mix of
regions with different compositions because the nucleus is actually
composed of smaller "mini-comets" (cometesimals), each with a different
chemistry. Deep Impact could have just so happened to hit one of these
cometesimals, while the gas seen before impact might have came from a
different region on the comet with different chemistry. Multiple impacts
in different regions of the comet would be necessary to determine which
scenario is correct, according to the team.

If the first scenario is correct, the comet could have formed in the
region now bounded by the orbits of Uranus and Neptune, based on its
interior chemistry. Different chemicals get frozen into a comet
depending on its location. A comet that forms farther from the sun will
have greater amounts of ices with low freezing temperatures, like
ethane, than a comet that forms closer to the sun. By measuring the
relative amounts of each chemical, astronomers can estimate where a
comet formed.

Formation in this location supports a theory that the gas giant planets
Uranus and Neptune formed closer to the sun than their current
locations. The theory, proposed by Dr. Alessandro Morbidelli of the
Observatoire de la Cote d'Azur, Nice, France, and his team, says that
gravitational interaction between the gas giant planets and numerous
small planets left over from the solar system's formation
(planetesimals) brought the giant planets into an unstable orbital
configuration. Neptune and Uranus were tossed outward and could have
exchanged orbits. As they migrated outward, their gravity disrupted a
large disk of comets that had formed in the region where Uranus and
Neptune currently reside. Some were scattered into deep space, to a
roughly spherical region called the "Oort cloud" that surrounds our
solar system at about 10,000 times the earth-sun distance. Others were
directed to the Kuiper belt, a region beyond Neptune that extends to
several hundred times the Earth-sun distance.

If some Kuiper belt comets have similar chemistry to some Oort cloud
comets, it would support this model of the solar system's rowdy early
days by showing that certain comets had a common origin despite very
different final destinations. Tempel-1 shares certain orbital
characteristics with the "ecliptic" comets, a group that likely comes
from the "scattered" Kuiper belt. "The amount of ethane in Tempel-1,
however, is similar to the amount in the dominant group of comets that
come from the Oort cloud region," said Mumma. Its chemical similarity to
Oort cloud comets supports the idea that some Kuiper belt and Oort cloud
comets formed in the same place.

This research was funded by NASA, the National Science Foundation, and
the National Research Council. The team includes scientists from NASA
Goddard, Rowan University, Glassboro, N.J., University of Toledo,
Toledo, Ohio, Kyoto Sangyo University, Kyoto, Japan, Johns Hopkins
University Applied Physics Laboratory, Laurel, Md., University of
Missouri - Saint Louis, and the W. M. Keck Observatory, Kamuela, Hawaii.
Received on Tue 20 Sep 2005 11:46:17 AM PDT